The present invention relates generally to methods and apparatus for animation in a movie system and more particularly to a method and apparatus for smoothing animation effects by compensating for arc-length variations in motion as an object travels along a path.
Programmed computers can be used to edit and create movies. For example, a computer program product called After Effects, available from Adobe Systems Incorporated of Mountain View, Calif., is licensed for use on a variety of personal computers and provides its user the ability to edit and create movies by integrating and arranging pieces of footage. In such programs, the process of creating a movie typically occurs in two major stages: modeling and rendering.
Modeling is the process of creating the structure for a movie project, commonly called a "composition", by defining the arrangement and timing of imported footage. A composition is essentially a set of instructions that define the processing of footage pieces in space and time in the formation of a movie. Each composition typically includes the definition of one or more layers, which are place holders for pieces of footage. Modeling includes the subprocesses of importing footage into the layers in a composition, editing the footage, arranging or "compositing" the various pieces of footage, and adding animation or other effects to the composition layers.
Imported footage may be in the form of video, pictures, animations, drawings, stills, photographs or computer generated images. Each piece of imported footage is assigned to a layer. Compositing integrates or combines the footage of the respective layers by using geometry masks, transparency information and effects. As the layers of the composition are integrated in the composition, animation and other effects are applied to each layer.
To create a final output, such as a film or videotape for viewing, the composition must be rendered. The rendering process transforms the footage and instructions associated with each layer into finished video frames. During the rendering process, corresponding pixels from each layer are composited on top of each other to create a final image, a frame at a time, in the output format requested by the user. The frames may then be written for either analog or digital storage on a recording device such as a video tape recorder, photographic film recorder or digital disk recorder. In this way a movie is produced.
Animation, as described above, is a subprocess of the modeling stage. Animation techniques allow a user to create apparently spontaneous, lifelike movement of objects in the composition. Most movie systems allow a user to animate an object (i.e., a layer) by specifying a path of the object as it moves through a two- or three-dimensional space. The path is typically represented as a spline curve Q. Motion along this spline curve Q may then be described by a single function (e.g., u as a function of time, where u is the natural parameter of the function defining Q)